So many responses! And theories galore—everything from tracheal mites to alien abduction. I promised to give my own opinion on the cause of death, but please remember that there is a lot of experience reflected in the posted answers. My own opinion is just that, so you should not discount what others have to say.
Several of you wished you had more information about the history of the colony, its size, mite treatments, last queen sighting, etc. I agree those things would be helpful, but knowing nothing is a common starting place. Detectives in a real crime scene usually begin with no history, and hive detectives often have the same handicap.
The final cause of death
My own conclusion is that the final cause of death—the thing that took out the last tiny cluster—was cold. As you know, a cluster of bees stays warm by vibrating their wing muscles to create heat, and the warm bees on the inside rotate positions with the cold bees on the outside. Aaron’s tiny cluster is too small to have an inside and an outside, so all the bees are going to be cold.
Once the bees become chilled, they can’t move, which renders it impossible to retrieve food even if it is close. So you could argue starvation, but in this case I’d say the two conditions were related: the bee cluster was too small to stay warm or to get stored food.
The real cause of death
Nevertheless, the real cause of death occurred much earlier. The few bees shown in the photos were just the hangers on; they were doomed to die and it was just a matter of time.
Trying to figure out why the colony collapsed in the first place is much more difficult. My thought was that the queen died in fall. I say this because there was a fair amount of honey left in the hive. The colony had to be robust enough to defend itself from robbers and yellowjackets right up until the end of flight season.
If the queen had died (or stopped laying) earlier, the colony may have weakened before cold weather arrived, giving predators a chance to rob the hive. If nothing else, Aaron’s other hive would have cleaned it out. But based on the photos, I didn’t see any ragged comb which would indicate robbers or yellowjackets.
The colony had been fairly vigorous at one point because the brood area was large in some of the frames (based on darkened comb) and because there was a fair bit of honey collected in a year when honey collection was not generally good here in western Washington.
Too late for queen replacement
I concluded from this that it was a fairly normal colony that was doing fine until it went queenless in the fall. I assume the queen failed too late in the year for the colony to raise a replacement or, even if they did, it was too late to get her mated.
This leads to the next question: why did the queen die (or stop laying)? Of course, I don’t know. But a lot of queens die in the fall. In my opinion, queens with short lifespans are the result of genetic weakness due to inbreeding. Over and over, I find that queens raised locally outperform and outlive queens shipped from large-scale queen farms. I believe local queens are better adapted and have a more robust gene pool.
But I don’t know the origin of Aaron’s queens, so that is just a blind guess based on seeing lots of production queens die in the fall for no apparent reason.
The Varroa connection
Now, obviously, the bees could have died for other reasons. Based on Aaron’s testimony that he saw no K-wing or deformed wings, at first I thought Varroa was not the primary cause of death. On the other hand, this looks like a classic case of death by Varroa.
Varroa-infected colonies frequently die in the fall as the ratio of mites to bees increases. Also, colonies that collapse from Varroa frequently contain only a few dead bees but lots of honey. The bees that are not infected continue to remove the dead and defend the hive while the colony population dwindles. Although I didn’t see any guanine deposits in the photos, it’s possible they didn’t show up due to the camera angle.
I eliminated tracheal mites mostly because they aren’t very common at the moment. In fact, last fall my professor in a beekeeping class did an informal survey of beekeepers from all over North America and found that tracheal mites are rare—even the testing labs are seeing few cases. Many people think that all the miticides used against Varroa may have weakened the tracheal mite population to the point where they aren’t much of a problem.
However, they still occur in certain areas and there is nothing to say that newer, better, stronger tracheal mites will not show up once they develop resistance to all the commonly used miticides. This would not surprise me one bit. If you (Aaron) are worried about your other hive, you can send a sample to a lab for analysis, or I can do it for you.
Mold grows as the bees die
I have to agree with Aaron that I don’t see the mold as cause for concern. Mold is a result of colony death, not a cause of it. Also, there wasn’t much. If ventilation had been a problem, I wouldn’t be surprised to see mold covering every frame. But here, isolated spots of mold are growing on damp pollen and on dead bees. This occurred because once the colony died, the circulation caused by the bees’ wings stopped, and heat generated by the bees’ bodies disappeared. With no heat and no air movement, mold is inevitable.
No “for sure” answer
For what it’s worth, my conclusion is the colony died from a combination of a failed queen and a Varroa mite infestation. It’s very possible that the mite load caused the queen to die, although she could have been offed by poor genetics, injury, or disease. It’s also possible the colony would have died from the mites even if the queen were healthy.
The colony might have been saved by timely Varroa treatments of some type, and a fall brood nest inspection to make sure the queen was present and laying. With today’s queen problems, the fall inspection is imperative.
Again, I want to thank Aaron for allowing me to use his analysis and photos—what a gift! Some of us are reluctant to have our misfortunes pasted all over the internet so, Aaron, you are much appreciated.